Problem Statement
Boiling water reactor power densities have been limited in the past to less than 56 kilowatts per liter (KW/1), primarily as a result of their original designs. These designs constrain the power outputs of these reactors due to thermal limits and stability considerations. Thermal limits include the maximum linear heat generation rate and the minimum critical power ratio.
The maximum linear heat generation rate (MLHGR) is that maximum amount of heat output by a lineal foot of fuel rod. Normal MLHGR rates for a boiling water nuclear reactor are in the range of 12.1 to 14.4 Kw/ft or, in pure metric units, 40 to 47 Kw/m. Simply stated, the MLHGR is a limitation established by the fuel pellet swelling and establishing a mechanical interference with the cladding containing the fuel rod. The MLHGR cannot be exceeded at any individual fuel rod within a fuel bundle without potential damage to that particular fuel rod within the fuel bundle. As no individual rod is permitted to be damaged within a fuel bundle, the entire bundle is limited in its performance to maintain the maximum linear heat generation rate in any given fuel rod location. It is to be understood that to the extent a particular bundle constituting part of a reactor core is limited in its output, the entire core is likewise limited.
The minimum critical power ratio (MCPR) is the ratio of that level of fuel bundle power at which some point experiences transition from nucleate to film boiling compared to the then present output of the fuel bundle. This ratio is not permitted to be less than a numerical value of one anywhere within an individual fuel bundle. If the limit were to be exceeded at any given location within the fuel bundle, the temperature of the cladding of the fuel rod would rapidly increase due to increased resistance in the heat flow path from the interior of the fuel rod to the exterior of the fuel rod. Potential failure of the particular fuel rod cladding could follow.
The concept of a ratio is utilized in establishing limits of critical power within the fuel bundle. The ratio is maintained at a limit where operating conditions--both expected in normal operations and during anticipated abnormal operating occurrences or "transients"--can occur without running the risk of damage to the sealed fuel rods within the reactor.
In already designed nuclear reactors, these thermal limits are largely established by the original design. There is, however, a need to increase the power output density of nuclear reactors of new manufacture.
Accordingly, the factors relating to the power output densities will be briefly reviewed. Conventional fuel designs will be briefly discussed, especially in so far as they incorporate many heterogenous distributions in their neutron density and related power output. Thereafter, reference will be made to certain new reactor designs.
Regarding the factors relating to increasing power densities, vessel sizes are limited in diameter to approximately seven meters, given the desire to continue to use forging to manufacture such vessels in existing manufacturing facilities capabilities. There exists a reluctance to expand vessel fabrication facilities beyond existing size limits under present market realities. Therefore, each reactor vessel is practically limited in its diameter. This requires that the number of fuel bundles within a BWR core is therefore limited.
Limitations also exist in establishing the active fuel length of fuel rod bundles since as fuel rod length increases, thermal margins and stability become of concern. The longer the fuel bundle, the greater the possibility of transition boiling unless considerable additional inlet coolant flow is provided. This is aggravated, however, by the higher fuel bundle pressure drop associated with increased length inlet flowrate. Further, stability at certain power rates requires rods be maintained short. If the boiling length is too long and the two phase pressure drop too high, thermal-hydraulic, and thermal-hydraulic-nuclear instabilities arise. As a practical matter, the active fuel length is limited to about 12.5 feet or, 3.81 m using metric units.
Once it is understood that both vessel diameter and fuel rod length are limited as a practical matter, it becomes clear that the total reactor volume available in any given reactor vessel approaches a limit. Therefore, the practical volume limit for a reactor is about 100,000 liters.
When a reactor is built, many costs are fixed and constant regardless of the power output of the installed plant. If the installed plant can have a higher power density, these fixed and constant costs become substantially more efficient.
Thus, there is a need for a new fuel design approach with potential to achieve higher power density to reduce the capital costs of nuclear reactors. This will enable any given reactor to have higher power output.
The forced circulation boiling water reactor is one alternative reactor that is able to achieve the higher power density requirements. Simply stated, such reactors--by forcing the flow of coolant along internal paths--have the ability to concentrate more power in a given plant location.